Up until a few years ago mobile to me meant working on 2metres through a repeater or occassionally simplex. However having spent a portion of my professional life working mobile albeit on X band through a satellite, something which is not for the faint hearted or small bank balance, I decided it was time I tried working HF mobile. My first attempt was a base loaded 80m monopole. This was only 1.3m long and moun ted on top of my vehicle.
This as can be expected did not prove to be very succesful. In fact, althoughI did get one report from a short wave listener, I made no real contacts with it. This lead to further reseach. Having read an article on the internet about a centre loaded 20m antenna, I decided to try the same techinque for a 80m antenna. The details of how this antenna was made is detailed below.
The First Antenna
In the begining
At the start let me say that the antennas that I have designed and built, using components that can be readily purchased from the likes of Maplins and your local DIY shop and can be made for a few pounds. See building tips
The antenna is made in four sections which consists of a base matching coil, a helical section, a centre loading coil and the top whip section with its capacity hat.
1.The Base Loading Coil
The matching coil is 20 turns of 20swg enamel copper wire spaced 2 mm apart between windings, wound on a 42mm OD PVC water pipe. The coil is tapped at 10 turns which acts as the 50 Ŕ feed point. The bottom of the coil is attached either to a mounting stud such as a M10 bolt or as I did an old SO 239 (Male) connector, potted into the tube.
A wire is then attached from the 50 Ŕ feeder to the tapped point on the coil. Using a 37mm to 13mm pipe adaptor the middle section can be fitted into the bottom section.
2. The Helical Section
The lower verticle section is something of an experiment. I thought about how I could make the load coil appear to be electrically higher than the physical height. I therefroe came up with the idea of a helically would element. By winding the helical with a wide spapcing between the turns would mean mean that it had very little inductance so was not adding to much to the load. It also meant that I could use some ordinary plastic water pipe to make the element.
The helical section is a 13mm OD plastic overflow tube 940mm long with a helical winding of 14swg enamelled copper wire wound up the tube with 33mm spacing between each winding. This is soldered to the base matching section.
3. The Centre Load Coil
The centre loading coil attaches to the helical section using the same 13mm to 37mm pipe adaptor as was used on the bottom section. The coil is wound again on 42mm OD PVC water pipe, about 200mm long. The coil consists of 80 turns of 20swg close wound on the tube. The top of the coil is attached to the whip using a ferrule and screw arrangement. This was an old 2m whip interface which I adapted fro my purposes.
4. The Top Whip
The top part of the antenna is a standard 1.3m stainless steel whip. I used an old 3/8 2m whip, but stainless steel sections can be picked up from rallies quite cheaply. This will need adjusting to bring the antenna to the frequency area desired.
One piece of essential equipment is the MFJ 259 Antenna analyser. This last part must be carried out on the vehicle you wish to mount the antenna and with all the necessary earth bonding wires in place as these will all affect the resonant point of the antenna.
What I found best was firstly to check at what frequency the antenna was initially resonant. This wants to be about 4MHz. If the antenna goes off low e.g. 3.2MHz then take a few turns of the centre loading coil until a tune of about 4MHz is reached. Then construct a simple X shaped capacity hat about 150mm across from two pieces of 16swg tinned copper wire and fix this about 2/3 of the way up the 1.3m whip. This should bring the tune down from 4MHz to about 3.5MHz. Carefully trim each arm of the capacity hat until the desired operating frequency is reached e.g.3.66MHz. Minor adjustments to the antenna can be made with a suitable ATU. I use my antenna from 3.6 to 3.79MHz using an MFJ mobile ATU.
Once this has been completed then check all the sections are well soldered, that the pipes are glued together and any other fixtures are well secured. Cover the exposed sections of the coils in PVC tape and spray a clear lacquer over the coils to improve the water-proofness. Then your antenna should be complete and happy mobile.
6. First Contacts
Following the lack of success from the original base loaded antenna I was very dubious about the outcome from this antenna. Recieve signals seemed good and the antenna tuned up well. I might add that I did not at this time have a suitable mobile transceiver so used my base station rig the FT 102 with my car parked on my driveway.
My first station worked was in Northern France, the second was in Holland and may more followed that. Having managed to prove that the antenna worked I then managed to borrow an Alinco DX70TH ( which later I purchased) and then started being truely mobile. Many contacts both truely mobile and static mobile folllowed over a period of a year.
7. The Future
The success of the 80m antenna then generated 40 and top band versions. All worked very well with contacts mobile on top band all over the UK. The 40m version worked well into both interUK and the near continent. More research followed where I looked into the idea of high Q coils similar to the Bug Catcher.
The problem that I could see with the Bug Catcher was the size and weight. The weight meant that if used mobile then a stong lower section the the antenna was going to be required and possibly some guying support. I needed to find a way of winding a high Q coil in the same way as I had made the load coils for the first antennas.
Whist on a trip to my local Wickes DIY store, I noticed that they sold a wide diameter tube which is used inside a tumble dryer. The tube is 100mm diameter, made of thin plastic and I thought ideal for winding a loading coil.
The construction of this antenna is explained below.
The Second Antenna
1. Base Matching Coil.
This consists of 20 turns of 18swg (16awg) (approx 1.25mm dia. Wound on a length of 42 -43mm diameter plastic water pipe. The coil length is approx 60mm long so the spacing between turns is about a wire thickness. The coil is tapped at turn 10 up from the base. This point however is very dependent on the mounting position on the vehicle which can be affected by local metalwork. During final tuning it may be necessary to move this point up or down.
The base of the coil is at electrical ground. I have glued a PL259 connector into a plastic former which fits into the inner diameter of the water pipe I often cut pieces out from a plastic kitchen chopping board which I can buy from my local super market. This board is made from polypropylene and is about 8mm thick. I use it a lot especially for making end caps and discs for the pipes.
So the centre pin of the Pl259 is attached to the tap point and the outer soldered to the base of the coil. In this picture you can see the slight bulge where the tapping pint is. The upper end of the coil is attached to the lower element
2. Lower Element
This is 1.2metres long and made from 15mm diameter aluminium tube. The diameter is not too important but should be strong enough to take the loading coil weight and bending force when used mobile.
A plastic plug is fixed into the top of this tube drilled a tapped to take a 6mm bolt which is used to attach the loading coil to the lower element.
3, Loading Coil.
This consists of 30 turns 18SWG (16awg)(1.25mm dia) enamel copper wire wound on a 100mm diameter plastic former and is 65mm long. The spacing between turns is between 1.5 a 2 wire diameters.
The former was made from a pipe which is used in the exhaust section of a tumble dryer. Use white plastic as black plastic can be carbon loaded and therefore lossy. This is a rigid tube used inside the tumble dryer and is about 500mm long, not the flexible stuff that goes from the back of the tumble dryer to outside or vent.
The cross braces for the coil again were made from the plastic chopping board. The former length is about 80 to 90mm to allow fixing of the cross braces. Attach the lower coil end to the lower element. I have drilled and tapped a 3mm dia hole in the tube to attach the end of the wire around it then screw it up tight.
In late versions I have used tinned braid about 1cm wide as can be seen in the photo below. This is more flexible than the copper wire used on the windings and so is easier to fix to upper and lower element.
The coil can be covered in PVC tape or heatsgrink to stop corrosion.
The upper end of the coil is wrapped round the 5mm bolt used to connect the load coil to the upper element as shown in the photograph.
4. Upper Element
I have tried 2 upper elements both of which work well. One is a 1.3m telescopic element bought from my local electronics store, the other is a 1.3m stainless steel whip section from an old 5/8th 2m whip. The advantage of the telescopic whip is that itís length can be altered to tune the antenna across the band. It also has a pre-drilled 4mm dia screw thread to attach it to the loading coil. However I have drilled this out and re tapped to take a 5mm screw which is stronger than the 4mm screw.
Again strength being essential for good mobile operation.
I have a bracket already attached to the underside of my car. I have made sure there are good earth straps across the door hinges and bonnet hinges, as well as a good earth strap to the antenna base fixture. I use 1cm wide braiding straps but you can use good high amperage electrical house wire.
I assemble the component parts of the antenna together and mount it to the antenna fixing bracket. I carry out tuning checks using an MFJ 259 analyser. I suggest you add a few extra turns to the loading coil at first as check at what frequency it tunes. It should go off low. Then remove turns to bring the antenna up to the desired frequency. If you are using the telescopic whip you can alter its length to bring it to the desired frequency. I also use a small ATU unit to give me wider band coverage but the basic antenna is tuned to approx 3.650MHz.
6. And Finally.
Once the antenna matches at the desired frequency then make sure all the joins and brackets are secure a tight, use glue to add strength where necessary. Wrap exposed connections with PVC tape, and if necessary around the coils themselves. I also usually overspray the coils with clear lacquer varnish. This helps glue the wires in place on the former, as well as adding additional waterproofing. .
Like the original 80m antenna this has made many good contacts both inter UK and to the near continent.